PCR can be wonderfully reliable when every part of the reaction is working. It can also be frustrating when a gel shows a faint smear, missing band, extra bands, or no product at all. The good news is that most PCR problems come from a small set of causes: template quality, primer design, cycling conditions, reagent balance, or contamination. A useful PCR troubleshooting plan starts with the pattern you see. A clean but weak band points to a different issue from several unexp